1. Field of the Invention
The present invention relates in general to a braking system for automotive vehicles such as four-wheel automobiles, two-wheel motor cycles or various vehicles or trucks for industrial purposes. More particularly, the invention is concerned with an anti-skid braking system capable of applying brakes to the wheels of the vehicle, while preventing skidding of the wheels on the road surface.
2. Discussion of the Prior Art
The friction force between a wheel of the vehicle and the road surface is maximum when the ratio of slip of the wheel on the road surface is in a certain range. In other words, it is desirable to control the brake force of each wheel so as to maintain its slip ratio within an optimum range, for stopping the vehicle with a minimum running distance after the brake application, without a loss of straight-line running stability of the vehicle. To this end, a variety of anti-skin braking systems are proposed. An example of such an anti-skin braking system is disclosed in Japanese Patent published for opposition purpose under Publication No. 51-6308. In this type of anti-skid braking system, the controller estimates a slip ratio of a wheel of the vehicle, based on an output signal of a speed sensor for detecting the rotating speed of the wheel. The fluid pressure in the brake cylinder for the wheel is controlled so that the slip of the wheel is held within a predetermined optimum range. While it has been stated above that "the controller estimates a slip ratio", the slip ratio is not actually estimated in the strict sense. Generally, the controller estimates the running speed of the vehicle (usually expressed by the rotating speed of the wheels), and merely obtains a difference between the estimated running speed of the vehicle, and the actual rotating speed of the wheels. In this respect, it is noted that the term "slip ratio" used herein is interpreted to also mean the "difference" indicated above.
It will be understood that the friction coefficient of the road surfaces on which the vehicle is expected to run is changed over a wide range, from an extremely low value as a frozen road surface, up to a very high value as a dry asphalt pavement. To cover such a wide range of friction coefficient of the road surfaces, it has been necessary to use a controller which is capable of controlling the braking pressure over a correspondingly wide range. Practically, however, a controller capable of achieving an optimum control of the braking pressure over such a wide range is difficult to design and manufacture. In the conventional anti-skid braking system, therefore, the braking pressure is controlled in a satisfactory manner in a relatively limited area of the range that should be covered. Namely, a satisfactory control of the braking pressure cannot be obtained in the area near the upper or lower limit of the pressure coverage. For example, a stable anti-skin control of the braking system may be attained while the slip ratio of the wheel is below the upper limit of an optimum range, but the stability of the anti-skid control is rapidly lowered when the slip ratio exceeds the upper limit. Hence, it is desired to rapidly reduce the fluid pressure in the brake cylinder when the slip ratio exceeds the upper limit of the optimum range. However, a rapid decrease in the fluid pressure makes it difficult to accurately control the amount of decrease of the fluid pressure. Further, since the rate of decrease of the fluid pressure in the comparatively high range is different from that in the comparatively low range, an optimum control of the pressure reducing rate is difficult to obtain over the entire range of the fluid pressure.